Most conventional magnetic recording media are the coated type prepared by forming a coating of powdered magnetic material on a non-magnetic base and drying the same. Examples of the powdered magnetic material are magnetic oxide particles or ferromagnetic alloy particles such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, and CrO.sub.2. These particles are dispersed in an organic binder such as a vinyl chloride-vinyl acetate copolymer, styrene-butadiene copolymer, epoxy resin or polyurethane resin. With the recent increasing demand for higher density recording, there has been increased interest in a magnetic recording medium of "thin metal film type" that eliminates a binder and uses, as a magnetic recording layer, a thin ferromagnetic film. The thin ferromagnetic film is formed by vapor deposition techniques such as vacuum deposition, sputtering and ion plating or plating techniques such as electroplating or electrolessplating. Various efforts are being made to use such recording medium on a commercial scale.
In most coated type conventional magnetic recording media, the magnetic material is made of a metal oxide having a small saturation magnetization. Accordingly, the coating must be above a certain minimum thickness required for high density recording, below which a drop in signal output occurs. In addition, the manufacturing process is complicated and requires large separate facilities for solvent recovery or pollution control. The great advantage of the magnetic recording medium of thin metal film type is that it can be prepared without using non-magnetic substances such as a binder in the formation of a magnetic layer and, as a result, the magnetic recording medium of thin metal film type allows markedly higher density recording than does the magnetic recording medium of coated type. However, this type of magnetic recording medium has a significant problem arising from corrosion, impact and friction resistance. During recording, reproduction and erasure of magnetic signals, the medium is placed in relative movement with the magnetic head and it wears or breaks due to contact with the head. Due to the absence of a binder, scratches are formed in the magnetic recording medium of thin metal type as it moves in slidable contact with the magnetic head. Accordingly, the magnetic recording is easily scraped off the medium. An attempt to solve the problem has been made by forming an overcoat about 0.2 .mu.m thick made of a polymer film. However, due to spacing loss, this causes decreased output during high-density recording.
It is known that the formation of scratches can be prevented by applying a thin coating of lubricant on the tape surface. The libricant reduces friction between the magnetic head and thin metal film and makes the film scratchproof. However, the effect of the lubricant does not last long and as the magnetic tape is used repeatedly, the friction between the head and thin metal film increases suddenly or the film breaks. An alternative method for reducing friction involves forming a lubricant protective layer of metal or metal oxide on the surface of magnetic tape. This method is described in Japanese Patent Application (OPI) Nos. 39708/78 and 40505/78 (the symbol OPI as used herein means an unexamined published Japanese patent application). But again, the effect of the lubricant protective layer does not last long, and as the magnetic tape is used, the friction increases suddenly or the magnetic film breaks.
In general, the thin magnetic metal film is formed on a very smooth base to achieve high-density recording. However, such smooth base is still entirely unsatisfactory for producing good running properties, particularly in a humid atmosphere. The base is also unsatisfactory for obtaining high abrasion resistance if it is prepared by the above-described methods used for increasing the lubricity of the magnetic layer.